The first gradual solar energetic particle event with enhanced 3He abundance on Solar Orbiter

TL;DR

This paper reports the first observation of a gradual solar energetic particle event with significantly enhanced 3He abundance by Solar Orbiter, suggesting residual 3He ions from prior events as the likely cause.

Contribution

It presents the first detailed analysis of a gradual SEP event with high 3He levels, linking it to residual ions from previous 3He-rich events, a novel insight into SEP composition.

Findings

3He/4He ratio was 24 times higher than typical solar wind values

Enhanced 3He likely originated from residual ions of a prior SEP event

Event associated with a fast halo CME observed by Solar Orbiter

Abstract

The origin of 3He abundance enhancements in coronal mass ejection (CME)-driven shock gradual solar energetic particle (SEP) events remains largely unexplained. Two mechanisms have been suggested - the re-acceleration of remnant flare material in interplanetary space and concomitant activity in the corona. We explore the first gradual SEP event with enhanced 3He abundance observed by Solar Orbiter. The event started on 2020 November 24 and was associated with a relatively fast halo CME. During the event, the spacecraft was at 0.9 au from the Sun. The event averaged 3He/4He abundance ratio is 24 times higher than the coronal or solar wind value, and the 3He intensity had timing similar to other species. We inspected available imaging, radio observations, and spacecraft magnetic connection to the CME source. It appears the most probable cause of the enhanced 3He abundance are residual 3He ions remaining from a preceding long period of 3He-rich SEPs on 2020 November 17-23.